Reflow soldering is employed extensively in the surface mount industries and particularly in the automated manufacture of printed circuit board assemblies. Generally, miniature electronic components are surface mounted on a printed circuit board to which a solder in a creamy or paste-like consistency has been applied by a method, such as screen printing, stenciling or dispensing.
The printed circuit board is then subjected to a sufficiently high temperature, generally 30.degree.-50.degree. C. greater than the melting point of the alloy, to cause the flux and the alloy in the solder to liquify and to contact the component so that upon subsequent cooling of the printed circuit board, the components are soldered in place on the board. The heat can be supplied by, for example, infrared, vapor phase, heated conveyor belt (hot belt) or convective means.
The solder paste is conventionally comprised of a soft powdered metal alloy dispersed in a liquid medium containing a flux, an organic solvent, and a thickening agent specially selected to impart the desired consistency to the mixture. Ideally, the flux component should be noncorrosive, thereby yielding flux residues after completion of soldering, which are themselves noncorrosive and nonconducting. In practice, however, such is not the case. Rosin-based flux materials, such as abietic acid-based flux, are used in most commercially available solders specifically formulated for use in the surface mount industries. These fluxes commonly contain activators, such as halides, particularly bromides, which leave corrosive and conductive residues requiring expensive and time-consuming removal techniques. Conventionally, these removal techniques utilize organic solvents, e.g., fluorochlorocarbons which give rise to environmental problems. As an alternative, aqueous systems have been tried for residue removal. However, due to poor wetting, it is difficult to obtain the penetration necessary with such systems to achieve the required cleaning. Additionally, removal of flux residues is difficult, particularly from areas of printed circuit boards beneath the components soldered thereto.
Rosin-based fluxes have additional disadvantages whether or not they contain conventional activators. For example, corrosive, rosin-based flux residues tend to be sticky, thereby inhibiting the automatic testing of the circuits and proving aesthetically objectionable.
The use of rosin-based or mildly activated rosin-based flux-containing solders can also result in poor wetting by the solder of the substrate and of components to be soldered depending upon the degree of oxidation on metallized regions and the oxygen content in the atmosphere.
Flux residues tend to be hygroscopic, thereby causing spattering, and some fluxes also undermine solder joint integrity by mechanisms such as permitting alloy particles in the solder to migrate from the solder site, giving rise to the formation of numerous small discrete balls of soft solder around the soldered joint, in effect creating electrical short circuits. Therefore, the industry has come to no-flux or no-clean solders used in low oxygen soldering environments.
U.S. Pat. No. 5,076,487 discloses the use of low oxidizing atmospheres for reflow soldering processes. The low oxidizing atmosphere comprises an inert gas with hydrogen wherein the hydrogen reduces the oxidation potential of water vapor and oxygen.
U.S. Pat. No. 5,044,542 discloses a wave soldering process, in contrast to reflow soldering, in which a hood is placed over the solder pot and a shield gas is dispensed above and below the conveyor for the elements to be soldered.
U.S. Pat. No. 4,921,156 discloses an additional wave soldering apparatus with an extended travel path through an enclosed tunnel. A protective nitrogen gas is dispensed within the tunnel.
U.S. Pat. No. 4,491,610 discloses a chamber for curing flat substrates in which an inlet knife and an outlet knife are positioned at the outer perimeters of the chamber.
U.S. Pat. No. 4,696,226 discloses a furnace with fluid barrier curtains at the inlet and outlet ends of the furnace. The fluid barrier curtains provide laminar flow based upon the orifice and vaned exhaust of the fluid barrier curtain. A gas, such as hydrogen, is introduced into the main portion of the furnace, while a gas, such as nitrogen, is used for the fluid barrier curtains.
The article "Infrared Reflow Solder Attachment of Surface Mounted Devices" by David K. Flattery appearing in Connection Technology, February 1986, pp 24-29, discloses at p 28, 3rd column, that the use of nitrogen or nitrogen and hydrogen are known in reflow solder furnaces.
Despite the efforts of the prior art to provide various furnaces and wave soldering operations with inert atmospheres, the art of reflow soldering still experiences difficulties in efficient soldering operations, disturbance of soldered components prior to the solidification of solder by localized high velocity gas currents, the efficient utilization of inert gases, and the need to adjust inerting operations and retrofit existing air, based reflow soldering furnaces. The present invention overcomes the shortcomings as will be set forth below in greater detail.